If higher requirements in terms of operational, fire, and contact safety have to be met when supplying electrical equipment with energy, the network type of an ungrounded power supply system is used, which is also known as an insulated network or IT power supply system (French: isolé terre—IT). In this kind of power supply system, the active parts are separated from the ground potential, i.e. against ground. The advantage of these networks is that the function of the electrical equipment is not affected in case of a first insulation fault, such as a fault to ground or a fault to frame, because the ideally infinitely large impedance value prevents a closed circuit from forming between the active conductors of the network (outer conductors and neutral conductor) and ground in this first fault case.
The inherent safety of the IT power supply system thus ensures a continuous power supply of the equipment connected to the IT power supply system, i.e. of the loads fed by the IT power supply system, even if a first insulation fault occurs.
Hence, the resistance of the IT power supply system against ground (insulation resistance, also called insulation fault, insulation fault resistance or fault resistance in the fault case) is continuously monitored according to standard IEC 61557-8 by means of an insulation monitoring device IMD, which is connected between the outer conductors of the IT power supply system and ground, because another potential fault on another active conductor (second fault) would cause a fault loop and the resulting fault current, in connection with an overcurrent protection device, would lead to a shut-down of the installation and to a standstill of operation.
For example, if controlled drive systems are connected to an IT power supply system as equipment or loads, installation parts of the IT power supply system can also comprise a direct-current circuit in addition to the alternating-current feed, such as a DC intermediate circuit of an inverter for supplying power to a three-phase motor. Because of a poor state of insulation of the direct-current circuit, a direct current (DC) may run to ground, which will lead to a displacement direct voltage (displacement DC voltage) between the outer conductor and ground in the IT power supply system. This displacement direct voltage can have a disruptive influence on the operating behavior of the connected load, which, under unfavorable conditions, may cause the load to fail. The fact that displacement direct voltages of this kind can have a negative impact on operating behavior is also apparent from the relevant recommendations and standards (cf., e.g., MIL-STD-1399-300B, section 5.2.12), according to which equipment connected to the IT power supply system can only be subjected to a displacement direct voltage of a certain height without sustaining damage. In connection with a critical displacement direct voltage, even a direct current as low as 100 mA can lead to a malfunction of sensitive control circuits.
To counteract the disruptive influence of the displacement direct voltage on the equipment and to trigger a shutdown of the faulty installation part with the direct-current circuit and the connected equipment, solution approaches are known from the state of the art which try to register the hazardous direct (residual) current with the aid of AC/DC-sensitive differential-current measuring technology.
However, in this case, it must be taken into account at first that, unlike in grounded systems, the reliable function of residual current protection devices (RCDs) in the ungrounded power supply systems considered at hand largely depends on the distribution and size of the present network leakage capacitances. It cannot be ensured in all installations that the functionality of all installed residual current protection devices is checked after slight changes of the IT power supply system, such as after the connection of another load branch.
Moreover, it must be taken into account that leakage currents of up to 20 A (RMS) may occur in extensive 3-phase power supply networks because of the large network leakage capacitances (MIL-STD-1399-300B, section 5.1.2). This means that capacitive leakage currents in the size of 20 A (RMS) have to be tolerated without shutdown while another residual direct current of 100 mA is required to cause a quick shutdown of the faulty installation part causing the direct current.
In view of the requirements regarding shutdown times, the manufacturers and operators of the IT power supply systems use the properties of residual current protection devices as a point of orientation; i.e., the desired shutdown times are in the range of 40 ms to 500 ms.
If one attempts to solve the problem of measuring and detecting the hidden residual direct current by filtering, for example, the expected step response times of the filter are so large that the requirements regarding shutdown times cannot be met.
Practical measurements and verification through simulation further showed that in case of a first insulation fault, displacement direct voltages against ground may occur in a connected driving system whose voltage amplitudes, if short-lived, are not yet operationally hazardous to the driving system and not critical to the loads in the rest of the IT system even after longer exposure, which means that a quick shutdown of the driving system is unwarranted. A premature shutdown in case of a small displacement direct voltage would also excessively reduce the availability of the drive. However, increased stress and early failure of the motor may occur in case of a longer exposure of the driving system to the insulation fault for a period of several seconds or even minutes.
Thus, it is to be noted that the requirements to be met extend far beyond the possibilities of realization offered by differential-current measuring technology known from the state of the art.